Japanese Patent Application Publication No. 2001-186768 describes a DC-DC converter that boosts a voltage supplied from a direct current source and outputs the boosted voltage. This DC-DC converter includes a low potential wiring, a first high potential wiring (an input-side high potential wiring), and a second high potential wiring (an output-side high potential wiring). This DC-DC converter further includes two FETs (field effect transistors) of n-channel type, and a source of each of the FETs is connected to the low potential wiring. A drain of the first FET is connected to the second high potential wiring via a first diode. A drain of the second FET is connected to the second high potential wiring via a second diode. Moreover, this DC-DC converter includes a main reactor, a first sub-reactor, and a second sub-reactor. The main reactor includes a first terminal and a second terminal. The first terminal of the main reactor is connected to a diode bridge (a type of a direct current source) via the first high potential wiring. One end of the first sub-reactor is connected to the second terminal of the main reactor, and other end of the first sub-reactor is connected to the drain of the first FET. One end of the second sub-reactor is connected to the second terminal of the main reactor, and other end of the second sub-reactor is connected to the drain of the second FET.
The first and second FETs are controlled so as to be alternately turned on. When the first FET is turned on, a current flows through the first sub-reactor and the first FET. Subsequently, when the first FET is turned off, a current flows through the first diode due to an induced voltage of the first sub-reactor. A voltage of the second high potential wiring is thereby boosted. Moreover, after the first FET is turned off, the second FET is turned on. When the second FET is turned on, the current flowing through the first sub-reactor and the first diode stops, and a current flows through the second sub-reactor and the second FET. Subsequently, when the second FET is turned off, a current flows through the second diode due to an induced voltage of the second sub-reactor. A voltage of the second high potential wiring is thereby boosted. After the second FET is turned off, the first FET is turned on again. When the first FET is turned on, the current flowing through the second sub-reactor and the second diode stops, and a current flows through the first sub-reactor and the first FET. Such alternate turn-on of the first and second FETs boosts the voltage of the second high potential wiring.
Moreover, immediately before the turn-on of the first FET, no current flows through the first sub-reactor. Therefore, after the turn-on of the first FET, the current flowing through the first FET rises gradually. Accordingly, a switching loss at the turn-on of the first FET is suppressed. Moreover, immediately before the turn-on of the second FET, no current flows through the second sub-reactor. Therefore, after the turn-on of the second FET, the current flowing through the second FET rises gradually. Accordingly, a switching loss at the turn-on of the second FET is suppressed. Such a FET switching in a state where a switching loss is less likely to occur may be called soft switching.